1. Field of the Invention
The invention relates to molding masses which can be thermoplastically processed, which consist in great proportion of methyl methacrylate units and have ethylene as a comonomer.
2. Description of the Background
Molding masses of polymethyl methacrylate have been known for a long time and offer the advantages of good thermoplastic processability, high transparency, and good weather resistance. Such molding masses generally contain 80-99 wt.- % methyl methacrylate units, as well as a remaining portion of other monomer units such as acrylic acid esters or styrene.
The ceiling temperature Tc is that temperature of homopolymers at which polymerization and depolymerization are in equilibrium. It can therefore be used as a measure for the thermal resistance of a polymer. Pure polymethyl methacrylate has a relatively low ceiling temperature Tc of about 160.degree. C. and therefore a relatively low stability against thermal decomposition. In this connection, it is essential for good processing stability that the polymethyl methacrylate chains do not contain any weak points, such as those caused by recombination or disproportionation of PMMA radicals. As a rule, PMMA molding masses are therefore produced with as little initiator as possible, in order to achieve good processing stability. Polymethyl methacrylate chains which were terminated by means of a transfer step (DE 43 40 887) are particularly stable.
Since polymethyl methacrylate molding masses are generally processed at much higher temperature, for example at approximately 200.degree.-270.degree. C. in injection molding processes, they are stabilized by means of copolymerization with other monomers, for example. Styrene or acrylic acid esters, for example, are usual comonomers (Tc=275.degree. C. and approximately 400.degree. C., respectively).
A particular advantage of these monomers results from the fact that they can be copolymerized with methyl methacrylate relatively well. Copolymerization parameters in radical polymerization, for example for the system methyl methacrylate=M1, methyl acrylate=M2 are r1=2.15, r2=0.40, for the system methyl methacrylate=M1, styrene=M2, they are r1=0.45, r2=0.44 (see, for example, Brandrup, J., Immergut, E. H., Polymerhandbook, 3rd Edition, 1989, John Eiley & Sons, N.Y.).
If, on the other hand, the copolymerization parameters of ethylene with methyl methacrylate are considered, it becomes clear that these compounds can be copolymerized with one another relatively poorly. Brown and Ham (J. Polymer Sci., Part A, Vol. 2, p. 3623, 1964) indicate the following r parameters for the system ethylene=M1 and methyl methacrylate=M2, for a temperature of 150.degree. C.: r1=0.2, r2=17. This means that in! a methyl methacrylate/ethylene system, even if there is only a small amount of methyl methacrylate being offered, the MMA will be built almost completely into the copolymer. On the other hand, even at a polymerization temperature of 150.degree. C., for example, and a high pressure, a significant excess of ethylene will be required in order to copolymerize only small amounts of ethylene with methyl methacrylate.
Ethylene (meth)acrylate copolymers with methyl methacrylate portions of up to 60 wt.- % are known from Ratzsch (1971, Plaste und Kautschuk Plastics and Rubber!, 18, p. 402). Ratzsch describes copolymers of ethylene with increasing proportions of acrylates, including methyl methacrylate. According to Ratzsch, polymerization takes place at 1500 bar and temperatures of 200.degree.-250.degree. C. The copolymerization parameters under these conditions are about 0.17 for the ethylene and 18 for the methyl methacrylate, according to Ratzsch. Copolymers with approximately 60 wt.- % methyl methacrylate are classified as sticky. No information is given with regard to corresponding copolymers with higher proportions of methyl methacrylate.
Polymethyl methacrylate molding masses with corresponding commercially available molding masses with methyl methacrylate proportions of 80 wt.- % or more, which contain ethylene as the comonomer, are not known. This is probably primarily due to the disadvantageous copolymerization parameters, as indicated above, and due to the fact that ethylene must be processed as a gas under high pressure, which would require corresponding complicated systems.